To provide a separation method that can easily separate high-purity ammonium hydrogen fluoride from a solid mixture that contains at least the ammonium hydrogen fluoride and a silicon compound.

In the separation method, the ammonium hydrogen fluoride is separated from the solid mixture 100 that contains the ammonium hydrogen fluoride and the silicon compound. The separation method comprises: a preparation step of obtaining a first solution 201 made by dissolving the mixture 100 in a solvent 300; a first crystallization step of cooling the first solution 201 to precipitate a solid substance 400 having higher content rate of the silicon compound than that of the mixture; and a second crystallization step of cooling a second solvent 202 in which the solid 400 is removed to a temperature lower than the first crystallization step, to precipitate a solid 500 higher in content rate of the ammonium hydrogen fluoride than that of the mixture.